Tapped transformer system



y z. 0. ST. PALLEY 2, 40,983

TAPPED TRANSFORMER SYSTEM File'd Dec. 12, 1945 Irwvemtor: Zolcan OStDal Iey,

Patented May 4, 1948 .Zoltan 0. St. Palley, Pittsfield, Mass, assignor to General Electric Company, a corporation of New York Application December 12, 1945, Serial No. 634,573

11 Claims.

This invention relates .to electric circuits and more particularly to improvements .in doublefinger switching-type load-ratio-control circuits.

A load-.ratioecontrol circuit is a circuit for varying the ratio of a transformer while under load, and a double-finger switching circuit of this type is one in which -the transformer circuit is in effect forked .and the .two tines or branches of the fork contain separate switching mechanisms for. making selective connection with different taps on a transformer winding. .In certain positions of the system, known as halfcycle .or bridging positions, .the branches of the forked circuit are connected to different voltage taps. This tends to produce a circulating current and the value of this .current is limited by a reactor which can either be a mid-tapped reactor or -a split reactor whose .two halves are in .the two branches of the forked circuit. If it is a mid-tapped reactor the electrical mid-point is connected to the circuit at the junction of the branches of the fork, while if it is a split reactor its two halves are separated so that other circuit elements may be effectively connected inside the reactor, it being understood that the main circuit conductor which is divided to form the branches of the fork is connected inside 'or between the two halves of the reactor.

In the-operation of such a circuit the branches are alternately but never simultaneously opened by respective arcing duty switches or contactors. The branch which remains closed carries all the current at such times and the branch which has been opened is eiiectivelymoved from one tap to an adjacent tap during such times. In many such circuits each branch has both an arcing duty contactor and a so-called ratio adjuster switch which does the actual tap changing and which is not adapted to perform any arcingduty .or current interruption, these different types of switches usually being serially connected in each branch.

One of the major problems in such circuits is the shortness of the .life of the arcing contactors relative to the other parts of the system. By contact life is actually meant the number of operation which the arcing duty contactors can perform without failure, but as in most systems the operations take place .at a fairly uniform rate, the life can also be thought of in terms of time. Speaking generally, the interrupting capacity of a switch is measured in volt-amperes, the .amperes being the current which is to be interrupted and the voltage being the recovery voltage. The recovery voltage is the voltage which appears between the separating contacts following the first current zero after the contacts begin to separate. Thus, for any given switch, the higher the current the lower the voltage which can be interrupted and the higher thevoltage the lower the current which can be interrupted. Furthermore, for any given switch there is usually an optimum relationship of current and voltage which gives the maximum contact life of the switch, that is to say, at this optimum relation the arcing duty on the switch is a minimum from the point of view Of contact life.

In systems of the above-described type if the circulating current limiting reactor .is so proportioned that the operating life .of the arcing duty contactors is a maximum under normal load conditions, then under short circuit load conditions the recovery voltage will be so high that the contactor will not be able to rupture this voltage .and extinguish the arc. The reason the recovery voltage is high is because the short circuit current flowing through one-half of the reactor produces a relatively very high voltage drop which appears as a part of the recovery voltage of the arcing duty contactor. If, on the other hand, the reactance of the reactor is made low enough so that the recovery voltage does not exceed the maximum voltage interrupting capacity of the contactor under short circuit load conditions, then the circulating current will be so high that the useful life of the contactor is very materially shortened below its maximum possible, In most systems of this kind the reactance of the reactor is a compromise between that which will give maximum contact life and that which will insure that the recovery voltage under the worst short circuit conditions will not exceed the maximum voltage rupturing capacity of the contactor.

In some systems of this kind there is a third contactor which short circuits the reactor when one of its arcing duty contactors is open. (Hereafter, such condition of the system will be referred to as its quarter-cycle position.) This is done .primarily to improve the regualtion of the system by eliminating the voltage drop produced by the load current flowing in one-half of the reactor when the system is in its quarter-cycle position. Therefore, it will also reduce the recovery voltage of the arcing duty contactors. However, this system has the serious defect that if undersevere short .circuitconditions an arcing duty contactor does notinterrupt its circuit, then there will be a tap-to-tap short circuit through the are which will destroy the transformer windmg.

In accordance with this invention means is provided for changing the eifective value of the reactor to a lower but finite value when the system is operated from its half-cycle position to its quarter-cycle position. With such an arrangement it is possible to proportion the reactor so that the circulating current under normal load conditions gives maximum contact life of the arcing duty switches, while what may be called the circulating current at short circuit loads, that is to say, the circulating current which would exist if the arcing contactor did 'not interrupt its circuit and an arc between its contacts persisted, can be high enough to bring the recovery voltage below the maximum rupturing voltage of the contactor and at the same time low enough to prevent destruction of the main winding if for any reason the arc were not extinguished.

An object of the invention is to provide a new and improved electric circuit.

Another object of the invention is to provide a new and improved load-ratio-control system.

A further object of the invention is to provide a load-ratio-control system with a circulating current limiting impedance having multiple finite values.

An additional object of the inventionis to increase the operating life of the arcing duty contactors of a load-ratio-control system.

Still another object of the invention is to increase the short circuit interrupting ability of a load-ratio-control circuit.

Theinvention will be better understood from the following description taken in connection with the accompanying drawing and its scope will 'be pointed out in the appended claims.

In the drawing Fig. 1 illustrates diagrammatically an embodiment of the invention, Fig. 2 is a modification, and Fig. 3 is a detailed view of a suitable form of double arcing contactor for use in the circuits shown in Figs. 1 and 2.

Referring now to the drawing and more particularly to Fig. 1, there is shown therein a main transformer winding I which is connected in'a load circuit 2., The winding l is provided with at least two spaced taps 3 and 4. The load circuit 2 is forkedor branched at point 5, the right-hand branch, which is indicated generally at 6, serving to interconnect point 5 and tap 3,

and the left-hand branch, which is indicated generally at 1, serving to interconnect point 5 and tap 4. These branches are duplicates of each other. As shown in the drawing, the two halves of an iron cored reactor 8 are connected directly together and to point 5. This reactor may be either a mid-tapped reactor or a split reactor but its two halves are preferably closely interlaced so as to have a very low leakage reactance. Therupper end of the branch 6, as shown in the drawing, terminates in a movable ratio adjuster contact or finger 9 and the corresponding end of the branch I terminates in a ratio adjuster or contact finger Ill and these two devices are ordinarily not adapted to interrupt any current or perform any arcing duty as they are ordinarily placed in the transformer tank and under the surface of the oil or other cooling and insulating liquid. The arcing or current interrupting duty of the two branches is performed respectively by arcing duty contactors II and I2. The arcing duty contactor II is shown in its full open position and an arc is indicated be substantially the same as the reactor 8 in that it has two equal windings. One winding I4 is connected between the branch circuit 1 and an auxiliary or back contact I5 on the arcing duty switch H, while another winding IE on the reactor i3 is connected between the branch circuit 6 and an auxiliary or back contact I! on the arcing duty switch I2. 7

The operation of Fig. 1 is as follows: With the ratio adjuster contact fingers 9 and It) in the position shown and with the arcing duty contactor! l closed in a position corresponding to that in which the contactor I2 is, the system is in its so-called half-cycle position. The reactor l3 will be completely out of the circuit and the reactor 8 will act as an autotransformer to give the point 5 a voltage half-way between the voltages of the taps 3 and 4. Any load current in the circuit 2 will divide equally through the two branches 6 and 1 and there will be substantially no load current produced voltage drop in the reactor 8 because the two halves of the reactor are so wound or connected that their magnetic effects cancel each other when load current flows through them. However, there will be a circulating current Io which will flow through the branch circuits 6 and 1 and the section of the main winding 1 between the taps 3 and 4. which is produced by the voltage E of the winding between those taps. Assuming that the reactor 8 has a reactance X to this circulating current, then it is clear that If now it is desired to change the voltage of point 5, one of the arcing duty contactors is first opened. For example, the contactor II can be moved to its open position and into engagement with the back contact l5, as shown in the drawing. This is for the purpose of interrupting the current through the ratio adjuster 9 so that it can be moved out of engagement with the tap 3 and into engagement with some other tap (not shown). Assuming that the frequency of the alternating current of the system is the usual sixty cycles, the current through the contactor will be passing through zero times a second so that the contacts will not separate very far before any are which is drawn goes out. This means that all of the load current will now flow through the branch 1, with the result that the reactor 8 is unbalanced as all of the load current is now flowing through one-half of the reactor 8. The voltage drop across this half of the reactor is, furthermore, doubled by the 2:1 step-up autotransformer action or connection of the reactor in this quarter-cycle condition of the system and this double voltage is added to the tap voltage 2 to produce the recovery voltage between the separating contacts of the switch II. It is thus clear that a very substantial part of the recovery voltage is determined by the value of the reactance X of the reactor 8. Typically the value of X may be so chosen that the circulating current in the half-cycle position of the system is onehalf of the rated full load current of the system.

Under ordinary or normal load conditions the are between the separating contacts-of the contactor ll will'not restrike. However, under short circuit load conditions in which the load current may rise to values of 'the'order of twenty-five times the normal full load current it is clear that the recovery voltage between'the separating contacts of the contactor -II will also increase in'theorder of twenty-five times that which exists under normal load conditions so'tha't it may very well exceed the maximum rupturing voltage of the contactor, in which case the arc will persist until the contactor reaches its full open position. However, in that position engagement with contact I effectively places the winding M in parallel with the reactor 8, thus connecting the winding 14 and the right-hand half of the reactor B in series with each other and in-parallel with theleit-hand half of the reactor 8 so far as the shortcircuit current in the load circuit is concerned and connecting the winding M in parallel with the entire reactor 8 so far as what may be called circulating current at short circuit load is concerned, that is to say, so far as circulating current which can flow through the arc between the contacts of the contactor H is concerned. Therefore, the recovery voltage is very materially reduced'below what it would be without the winding 14 in circuit because the reactor 8 becomes partially balanced or neutralized. Furthermore, the circulating current through the arc can be limited to a safe value. Thus, if the reactance of winding i4 is X0, then the circulating current at short circuit loads, which may be designated as In, will be'determined by the formula Byproperly choosing the'value of X0 the current I0 can be made of the order of 250 per cent of normal full load current, which of course will be very much below the current which would be produced by a dead short circuit between the taps other words, this connection reduces the regula tion 'of the system in its quarter-cycle position substantially below what it would be if all of the load current flowed through only one-half of the reactor 8.

The system shown in Fig. 1 is symmetrical and therefore the operation of the circuit elements in the branch I will be the same as for the branch 6, the only difference being that the contactor l2 when it is moved to the full open position will place thewinding l6 of the reactor l3 in parallel circuitrelationship with the reactor 8 so far as circulating currents and short circuit loads are concerned.

The modification shown in Fig. 2 differs from Fig. l-primarily in that the control of the effect'i've value of the circulating current limiting impedance is by 's'eries'circuit connections instead of by arallel circuit connections. Another'difference is that the main reactor '8 and the auxiliary reactor l3 have their positions effectively interchanged so that main reactort now becomes a split'reactor instead of amid-tapped reactor. Furthermore, the contactors l and 12 are so arranged that when they make contact with their back contacts'in their full open position they "respectively short circuit the difierent halves of the main'rea'ctor 8' which has theeffectof short-circulting the entire reactor because the volts per turnoi any turn on the reactor'will be held substantially to zero by the short circuiting of any otherturn or turnsthereon due to the very close coupling between alltheturns.

With'both contactors 'H and [2 in their closed positions, thatis-to say, with contactor-ll in the position in which contactor is shown, the reactors Band l3 are connected in series between the taps 3 and 4 so that the circulating current at'normal load conditions will be given by the formula where X0 in this case is the reactance of both of the windings l4 and 16 in series. Asin Fig. l, the value-of circulating current I -can be given any desired value by making the sum of X and X0 of the proper value without regard to circuit conditions under short circuit loads. However,

as soon as either contactor il or lireaches its full open position andengages its back contact one or theother halves of the main reactor 8 will be short circuited, thus effectively removing therreactor 8 from the circuit, thus'decreasing the recovery voltage between the-open contacts of the contactor and also increasing the circulat- 2| and I5 whichare also mounted on suitable insulating supports similar to the member 19.

Connections are madethrough thepanel board by the three leadsshown extending therethrough.

While'there have been shown and described particular embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications can bemade therein without departing from the invention and, therefore, it is aimed in the appended claimsto cover alls'uch changes and modificationsas. fall within the true spirit and scope of the invention.

What I claim as'new and desire to secure by Letters Patent of the United States, is:

1. A load-ratio-control system comprising, in combination, an arcing duty contacton connections for causing said contactorzto establish .a half-cycle operating condition of said system when said contactor is closed and for establishing a quarter-cycle condition of'saitl system'when said contactor is open, impedance means =whose Valuecontrolsthe amount of circulating current through said contactor when said contactor is closed and the amount of-recovery voltage of-said 7 contactor when said contactor is open; and means for varying said impedance means between a relatively high value when said contactor is closed and a relatively low but finite value when said contactor is open.

2. A load-ratio-control system comprising, in combination, an arcing duty contactor having a predetermined maximum voltage rupturing capacity, connections for causing said contactor to establish a half-cycle operating condition of said system when said contactor is closed and for establishing a quarter-cycle operating condition of said system when said contactor is open, impedance means whose value controls the amount of circulating current through said contactor when said contactor is closed and the amount of recovery voltage on said contactor when said contactor is open, said impedance having a normal value such that the operating life of said contactor under normal load conditions is a maximum, and means operative when said contactor attains its full open position for reducing the value of said impedance to such a value that the recovery voltage of said contactor under short circuit load conditions is just under said maximum voltage rupturing capacity of said contactor, said reduced value of said impedance being sufficiently great to prevent a tap-to-tap short circuit.

3. In combination, an electrical winding having I at least two taps, impedance means having two end terminals and an intermediate terminal, separate' circuit making and breaking means for respectively connecting the end terminals of said impedance means to said taps, an electric circuit for said winding including a conductor connected to said intermediate terminal, and means responsive to a circuit breaking operation of either of said circuit making and breaking means for decreasing the effective value of said impedance means to a lower but finite value.

4. In combination, a transformer winding having a plurality of taps, an electrically mid-tapped reactor, means including a pair of arcing duty contactors for respectively connecting the terminals of said reactor to different ones of said taps and disconnecting them therefrom, a circuit for said transformer including a conductor connected to the mid-tap of said reactor, an impedance element, and means associated with an opening operation of one of said contactors for connecting said impedance element in shunt circuit relation with said reactor and associated with a closing operation of said last-mentioned contactor for open circuiting said impedance element.

5. In combination, a transformer winding having a plurality of taps, an electrically mid-tapped reactor, means including a pair of arcing duty contactors for respectively connecting the terminals of said reactor to different ones of said taps and disconnecting them therefrom, a circuit for said transformer including a conductor connected to the mid-tap of said reactor, an impedance element, and means operative upon one of said contactors reaching its full open position for connecting said impedance element in shunt circuit relation with said reactor through said other contactor and associated with a closing operation of said one contactor for open circuiting said impedance element.

6. In combination, a transformer winding having a plurality of taps, an electrically mid-tapped reactor, means including a pair of arcing duty contactors for respectively connecting the terminals of said reactor to different ones of said taps and disconnecting them therefrom, a circuit for said transformer including a conductor connected to the mid-tap of said reactor, an impedance element, means associated with an opening operation of one of said contactors for connecting said impedance element in shunt circuit relation with said reactor and associated with a closin operation of said last-mentioned contactor for open circuiting said impedance element, a second impedance element, and means associated with an opening operation of the other of said contactors for connecting said second impedance element in shunt circuit relation with said reactor and associated with a closing operation of said other of said contactors for open circuiting said second impedance element.

7. In combination, a transformer winding having a plurality of taps, an electrically mid-tapped reactor, means including a pair of arcing duty contactors for respectively connecting the terminals of said reactor to different ones of said taps and disconnecting them therefrom, a circuit for said transformer including a conductor connected to the mid-tap of said reactor, an impedance element, means operative upon one of said contactors being actuated to its full open position for connecting said impedance element in shunt circuit relation with said reactor through the other of said contactors and associated with a closing operation of said one contactor for open circuiting said impedance element, a second impedance element, and means operative upon the other contactor being actuated to its full open position for connecting said second impedance element in shunt circuit relation with said reactor through the first-mentioned contactor and operative upon a closing operation of said other contactor for open circuiting said second impedance element.

8. In combination, a transformer winding having a plurality of taps, a main reactor having end terminals and an electrically mid-tapped reactor, means including a pair of arcing duty contactors for respectively connecting the end terminals of said reactor to different ones of said taps and disconnecting them therefrom, a circuit for said transformer including a conductor connected to the mid-tap of said reactor, an auxiliary two-winding reactor, means associated with an opening operation of one of said contactors for connecting one of the windings of said auxiliary reactor in shunt circuit relation with said main reactor, and means associated with an opening operation of the other of said contactors for connecting the other winding of said auxiliary reactor in shunt circuit relation with said main reactor.

9. In combination, a transformer winding having a plurality of taps, an electrically mid-tapped reactor, means including a pair of arcing duty contactors for respectively connecting the terminals of said reactor to different ones of said taps and disconnecting them therefrom, a circuit for said transformer including a conductor connected to the mid-tap of said reactor, an auxiliary two-winding reactor, means operative upon one of said contactors reaching its full open position for connecting one of the windings of said auxiliary reactor in shunt circuit relation with said mid-tapped reactor through the other of said contactors, and means operative upon said other contactor reaching its full open position for connecting the other Winding of said auxiliary reactor in shunt circuit relation with said one of said contactors.

10. In combination, a transformer winding having a plurality of taps, an electrically midtapped reactor, a pair of impedance elements, a pair of arcing duty contactors for respectively connecting the terminals of said reactor to different ones of said taps by way of a series connection through difierent ones of said impedance elements, and means associated with an opening motion of either contactor for efiectively short circuiting the impedance element in series therewith.

11. In combination, a transformer winding having a plurality of taps, a split main reactor, a pair of arcing duty contactors for respectively connecting the terminals of opposite halves of said reactor to different ones of said taps, an electrically mid-tapped auxiliary reactor having 15 Number 10 end terminals connected respectively to the remaining free terminals of said split reactor and having its mid-tap connected to a load circuit for said transformer winding, and means associated with an opening operation of either contactor for efiectively short circuiting one ofthe halves of said split main reactor.

ZOLTAN 0. ST. PALLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date 1,626,970 Rooth May 3, 1927 

